(1) Field of the Invention
The present invention relates to a wiring substrate that is used for mounting electronic components such as an optical semiconductor device, a manufacturing method of the substrate, and an electronic component device.
(2) Description of the Related Art
Conventionally, a build-up method has been widely used as a technology of manufacturing a wiring substrate in a multilayer wiring structure. Many types of the wiring substrate formed by using the build-up method can be fabricated by combinations of a material of an interlayer insulating film (resin, representatively) and a via hole forming process. Its typical manufacturing process is performed by sequentially repeating formation of an insulating layer, a via hole in the insulating layer and a wiring layer including the inner part of the via hole to stack them on both surfaces or one surface of a core substrate as a supporting base material. In such a structure, a portion composed of wiring layers and insulating layers can be formed thin because they are stacked by the build-up method, but the core substrate requires a reasonable thickness in order to allow the wiring substrate to have rigidity. Thus, forming the entire wiring substrate (semiconductor package) thinner is limited.
For this reason, in order to make the wiring substrate even thinner, a structure in which a core substrate (supporting member) is removed recently has been employed. A wiring substrate of such a structure is also called a “coreless substrate” in a sense of having no “core” portion.
According to a basic process in a manufacturing method of such a coreless substrate, a temporary substrate 1 as a supporting body is prepared, a required number of build-up layers (insulating layers 3, 5, 7 including via holes 3a, 5a, 7a, and wiring layers 4, 6, 8 including the inner part of via holes 3a, 5a, 7a) are sequentially formed in a wiring forming region 10 of the temporary substrate 1 as shown in the cross-section of FIG. 1A, and then, the temporary substrate 1 is removed as shown in the cross-section of FIG. 1B. Note that reference numeral 2 in FIGS. 1A and 1B denotes a lower wiring layer, 9 denotes a solder resist film, 11 denotes an outer periphery region surrounding a wiring forming region 10, and reference numeral 101 in FIG. 1B denotes a coreless substrate (wiring substrate).
The coreless substrate 101 in use is inverted up and down at the time of mounting a chip 13 as shown in FIGS. 2A and 2B. Note that reference numeral 12 in FIGS. 2A and 2B denotes a bump. FIG. 3 shows a top view of the coreless substrate 101, and the coreless substrate 101 of FIGS. 2A and 2B corresponds to a cross-sectional view taken along line I-I of FIG. 3.
Related art is disclosed in Japanese Patent Laid-open No. 2007-73766.
As described above, since the conventional coreless substrate 101 has no core substrate, it is advantageous in the point of forming the substrate thinner, but on the contrary, it is disadvantageous in the point of easy occurrence of a “warp” in the coreless substrate 101 because of low rigidity of the entire coreless substrate 101.
This appears more conspicuously when the chip 13 is mounted as shown in FIGS. 2A and 2B. Specifically, after performing heating and cooling processes in mounting the chip 13, as shown in the dotted line of FIG. 2B, a “warp” is caused to occur by a thermal expansion coefficient difference between the chip 13 and the coreless substrate 101. Furthermore, the “warp” is promoted by a thermal expansion coefficient difference between underfill resin 14, which is filled in a gap between the coreless substrate 101 and the chip 13, and the coreless substrate 101. For this reason, there are cases where the mounted chip is often cracked or peeled, and it causes a problem that reliability of chip mounting drastically reduces.
Further, the warp of substrate does not necessarily occur only when mounting a chip, but also it could occur on the stage before mounting a chip. For example, in the case where coreless substrates are shipped to customer's premises and chips are mounted in the customer's premises, the warp could occur in the substrate depending on handling from shipping to mounting because the rigidity of the coreless substrates is originally low and flexible.